The application area of chargeable and dischargeable secondary batteries is being increasingly expanded to electric vehicles as well as portable devices such as mobile phones, notebooks, and camcorders. Accordingly, secondary batteries have been actively developed. Also, research and development of battery design to improve capacity density and specific energy have been conducted during the development of the secondary batteries.
In general, it is known that battery safety improves in the order of a liquid electrolyte, a gel polymer electrolyte, and a solid polymer electrolyte, but battery performance decreases in the same order.
An electrolyte in a liquid state, particularly, an ion conductive organic liquid electrolyte, in which a salt is dissolved in a non-aqueous organic solvent, has been mainly used as an electrolyte for an electrochemical device, such as a typical battery using an electrochemical reaction and an electric double-layer capacitor. However, when the electrolyte in a liquid state is used, an electrode material may degrade and the organic solvent is likely to be volatilized. Also, there may be limitations in safety such as combustion due to ambient temperature and the temperature rise of the battery itself.
In particular, since an electrolyte used in a lithium secondary battery is in a liquid state and may have a risk of flammability in a high-temperature environment, this may impose a significant burden on electric vehicle applications. Since the above limitations may be addressed when the lithium electrolyte in a liquid state is replaced with a solid-state electrolyte, various conventional solid electrolytes have been researched and developed.
Among them, a perovskite-structure oxide having a chemical formula of Li0.33La0.66TiO3 (LLTO) is a material having high chemical stability and durability as well as excellent lithium ion conductivity.
Typically, in order to synthesize LLTO, lithium precursor, lanthanum precursor, and titanium precursor powders are mixed and heat treated at a high temperature of 1,200° C. or more for a long period of time, and a LLTO solid electrolyte is then prepared through a grinding process. However, in this case, the high-temperature and prolonged heat treatment process may be uneconomical in terms of cost. Also, since the LLTO solid electrolyte having a particle diameter ranging from a few hundred nm to a few μm is prepared, the contact area between electrolyte particles and electrode particles may not only be limited but there may be limitations in reducing the thickness of an electrolyte layer.
Furthermore, in order to decrease the particle diameter of the LLTO solid electrolyte having a particle diameter of a few μm, a method of grinding the electrolyte has been performed. In this case, the particle diameter may be decreased by the grinding, but there is a limitation in obtaining a uniform particle diameter distribution.